The present invention relates to an optical displacement detector or an optical encoder utilizing an encoder plate which is mounted to undergo an angular or linear displacement to modulate an incident light so that its displacement is detected based on variation in a modulated light intensity.
The conventional optical encoder utilizes either of a geometrical optics system and a wave optics system. The known optical encoder of the wave optics type utilizes an interference or a diffraction of coherent light, and comprises a coherent light source such as a semiconductor laser, an encoder plate formed with a diffraction grating and a photosensor. The diffraction grating has a lattice constant comparable to a wavelength of the coherent light, hence the optical encoder of the wave optics type features high resolution and compact size. However, the semiconductor laser generally utilized as the coherent light source suffers from a temperature-dependent oscillating characteristic which would cause a detection error. In addition, the semiconductor laser suffers from a relatively short life time.
On the other hand, the known optical encoder of the geometrical optics type utilizes a linearity or directibility of the light, and comprises an incoherent light source such as a light emitting diode (LED), a pair of a movable slit plate and a stationary slit plate, and a photosensor. The combination of the movable and stationary slit plates is utilized to intermittently switch an incident light so as to detect a displacement based on a light intensity variation.
The encoder of the geometrical optics system generally utilizes an incoherent light source composed of the light emitting diode LED, which has a life time longer than that of the semiconductor laser used as a coherent light source in the wave optics system. Further, the temperature dependency does not matter since the LED is utilized as an incoherent light source. However, in order to improve resolution in the encoder of the geometrical optics system, a pitch of slits formed on the movable and stationary slit plates must be reduced, thereby causing degradation of the directibility of the light transmitting through the slits due to optical diffraction. In addition, the LED is not an ideal point light source, but has actually a certain light emitting area to thereby cause divergence of the light transmitting through the slits. The moving slit plate and the stationary slit plate must be faced with each other as close as possible in order to prevent the degradation of light beam directibility and to prevent the light divergence. However, the moving slit plate constitutes an encoder plate which normally undergoes an irregular surface level fluctuation during the course of a regular movement. Such a surface level fluctuation is accelerated by mechanical vibration or shock imposed externally. Therefore, the gap between the moving slit plate and the stationary slit plate must be set in taking account of a certain allowance for the surface level fluctuation. Thus, it is practically difficult to suppress the degradation of the light beam directibility, thereby hindering fine resolution and scale-down of the geometrical optics type encoder.